1. Field of the Invention
The invention relates to digital imaging devices and, more particularly, to image sensing devices.
2. Description of Related Art
Image sensing devices are the light detecting components in digital imaging systems, such as for example, digital cameras and copiers. A digital image sensing device, such as a camera, uses light to capture an image typically by a semiconductor-based chip. The chip replaces film in traditional film-based systems. In a camera, an image sensing device is configured, in its simplest form, to capture a monochrome or color image by way of semiconductor devices such as transistors, capacitors, and photodiodes. In one example, the image sensing device is a chip made up of a number of pixels, each pixel capable of absorbing light. In color applications, each pixel generally absorbs light through a filter and represents one color corresponding to the image sensed.
In general, a pixel contains a photosensing structure, such as a photodiode, and other pixel circuitry. The photosensing structure is the region of the pixel that responds to light. In one example, a pixel circuit having a photodiode is charged to a predetermined voltage. The photodiode is exposed to light and a pixel circuit discharges its stored energy depending on the intensity of the light exposure.
The photodiode of a complementary metal oxide semiconductor (CMOS) imaging sensing array is typically reverse-biased. Under this condition, the photodiode is designed to block current flow as long as the voltage stays below a specified value. The specified value is exceeded upon the signaling of an electronic shutter. Under reverse-biased conditions, incoming photons strike a photodiode area and electron/hole pairs are generated through a junction in the semiconductor substrate. The electrons are collected as a signal representative of the light exposure. The signal gets transferred into image information upon the operation of the electronic shutter, e.g., sending a signal exceeding a predetermined voltage value.
One problem with many prior art photodiode architectures is that, under reverse-biased conditions, a small current, identified as a leakage current, generally flows across the junction. This electron flow acts as noise and may be referred to as a xe2x80x9cdark current.xe2x80x9d Under the conditions where the photodiode is not exposed to light there is a base line level of dark current equivalent to a reverse saturation leakage current. It would be desirable to reduce or eliminate this leakage current.
A circuit including an isolation boundary formed to a depth in a substrate defining an active area of the substrate, a primary junction formed in the active area to a primary junction depth in the substrate to collect electron/hole pairs, and a secondary junction formed in the active area adjacent the isolation boundary to a secondary junction depth at least equal to the isolation boundary depth.